There has so far been known an apparatus for removing dust in a dust-containing gas with use of filtration tubes made of a solid porous material as shown in FIG. 1. In FIG. 1, a plurality of filtration tubes 1 made of a solid porous material having gas-permeability are vertically arranged in parallel with each other, with their lower ends closed and their upper end opened. The filtration tubes 1 whose outer and upper ends are supported by a supporting plate 70 in a gas-tight manner, are received in a middle case 2b which is provided with a dust-containing gas inlet 3 in its side wall. A lower case 2c functions as a hopper for dust separation and is provided at its bottom part with a dust discharging port 5 and a discharging valve 5a through which dust separated is intermittently taken out. A clean gas outlet 4 is formed at the top of an upper case 2a.
In the operation of the conventional apparatus, a dust-containing gas fed through the dust-containing gas inlet 3 is introduced into a space outside the filtration tubes, and only a clean gas is caused to pass through the tube wall of the filtration tubes 1 from the outside of the tubes to the inside thereof due to pressure difference between the interior and the exterior of the filtration tubes. Thereafter, the clean gas is discharged through the clean gas outlet 4 out of the system. On the other hand, dust blocked from passing through the filtration tubes gradually accumulates on the outer wall of the filtration tubes with the lapse of time. The dust having accumulated on the outer walls occasionally falls on the bottom of the lower case 2c where it is collected, while the accumulated dust forms a filtration layer which performs a precise filtration for a dust-containing gas.
When the thickness of dust layer formed by accumulation of dust increases, the pressure difference between the exterior and the interior of the filtration tube becomes large. Accordingly, the dust layer on the filtration tubes should be removed by reverse cleaning at predetermined time intervals to reduce permeation resistance. Such dust removing apparatus is disclosed, for example, in "Wasser Luft und Betrieb" 16 (1972) Nr2 Page 61-62, titled "Verfahren zur Trockenreinigung von heissen Gasen"; "Chemical Engineering" Oct. 1981, Page 34-49, titled "Ceramic filter and application thereof"; Japanese Unexamined Patent Publication No. 3620/1983; and Japanese Unexamined Patent Publication No. 3621/1983.
Filtration of a dust-containing gas with use of a gas-permeable porous material is performed by using a surface filtration structure in which a dust layer accumulated on the surface of the porous material is used as a filtration layer. Accordingly, when a porous cylindrical body is used, it is advatageous from the viewpoint of pressure loss to use an outer wall having a large surface area as a filtration surface, but not an inner wall having a small surface area. In this case, even when dust enters into the outer surface of the filtration tube to a certain extent to cause abnormally large pressure loss in the filtration tube 1, it is easy to reuse it by scraping the outer wall of the filtration tube by a brush or a whetstone. Similarly, in a case using the outer wall of the filtration tube 1 as a filtration surface, even when dust enters inside the filtration tube to fill the pores, the dust can be easily removed by means of reverse cleaning (i.e. reverse flowing of a clean gas from the side of the inner wall) because distribution of the pores is substantially uniform in the radial direction of the filtration tube and the sectional area through which a gas is passed becomes large along the flow of the gas. In other words, the number of branches of open pores communicated from the inner wall side to the outer wall side increases toward the outer surface side of the filtration tube 1.
Although the conventional dust removing apparatus has advantage as above-mentioned, it has not been practised on a large size and in an industrial scale. The reason of this is considered as follows.
Use of a filtration tube made of a material such as ceramics is desirable under an elevated temperature condition. However, the length of a single filtration tube is limited to at most about 2 meters or so because the ceramic filtration tube before sintering generally has an extremely small strength. Accordingly, a filtration tube of a considerable length is apt to be deformed in the course of handling and sintering. There has been proposed a technique to bond with an adhesive filtration tubes in the longitudinal direction. It, however, causes difficulty in its bond strength. Further, there is another system as shown in FIG. 2 in which a plurality of filtration tubes are stacked in the longitudinal direction, a flange 31 is applied at the lowermost end to support a series of the filtration tubes, the flange is suspended with a metallic long bolt 33 which is inserted from the top of the series of filtration tubes to pass through the inside thereof, and a spring 32 is provided on the uppermost filtration tube to compensate the thermal expansion of the long bolt 33. The system has, however, many disadvantages. An excessive elongation of the long bolt 33 takes place when passing a high temperature gas; a creep and corrosion may be caused in the long bolt 33; the apparatus is unstable in structure against earthquakes and vibrations; and, if an arm (not shown in the drawing) is secured to the middle case 2b to support the flange 31 for a stable structure against the earthquakes and vibrations, dust tends to accumulate on such supporting means as the arm and remains on it. These problems have prevented use of long filtration tubes. Accordingly, the amount of gas to be treated per unit area for installation is extremely small, and the conventional apparatus has not been suitable for industrial use for treatment of a large amount of air.
Further, when a gas having a high dust concentration should be treated, since dust deposits on the outer surface of the filtration tube 1 except for relatively coarse particle dust which freely falls, increase of pressure difference between the inner and outer walls of the filtration tube is so rapid that it is necessary to carry out frequently reverse cleaning. This increases the cost of electric power for a blower for reverse cleaning, and an additional gas-treating apparatus should be installed taking into account of reduction in throughput at the time of the reverse cleaning. Accordingly, initial investment is large.